Thermoplastic polymers, because of their physical, chemical, and mechanical properties, frequently are used to make shaped articles such as, for example, extruded film, sheet, profiles, formed products, composite structures, and laminates. An important criterion for thermoplastic polymers used in such applications is the ability to maintain a desirable appearance over the lifetime of the article. In particular, it is important for the thermoplastic material to maintain color, clarity, resistance to haze, and to resist scratching by mechanical abrasion. One way to impart abrasion resistance is to coat the article with a tough, hard, and clear coating that resists scratching and hazing from abrasion and rubbing. In addition to abrasion resistance, such a coating generally will provide protection against weathering and chemicals such as, for example, solvents, polishes, and cleaners.
Common coatings that are applied to thermoplastic substrates include various thermoplastic polymers, unsaturated polyesters, epoxy resins, phenolics, melamines, acrylates, urethane-acrylates, rubbers, elastomers, and the like. In order for these coatings to be effective, they should be easy to apply to the thermoplastic article and should not degrade the physical or mechanical properties of the article such as, for example, its strength or toughness. Coating compositions have been developed which, when applied to a substrate and cured, impart a highly abrasion resistant surface to the substrate. For example, abrasion resistant coatings can be prepared from acrylate monomers and cured or crosslinked by radiation. These radiation-curable coatings can exhibit superior hardness, and abrasion and chemical resistance. Radiation curable coatings can be rapidly cured without the use of ovens, and can be applied and processed in the absence of hazardous solvents. Radiation curable coatings also have been used to impart abrasion resistance to plastic lenses such as, for example, eyeglass lenses, to plastic panels and films, and to wood and furniture surfaces.
Although radiation curable coatings are quite hard and resistant to abrasion and scratching, they often can cause cracking and brittle failure of the entire article when subjected to impact. One approach to this brittleness and cracking problem is to use a softening comonomer (a monomer with a low second order transition temperature) to impart some degree of flexibility to the coating. In achieving increased flexibility and reduced brittleness, however, the abrasion resistance of the coating can be reduced. A coating composition for thermoplastic substrates is needed, therefore, that provides excellent abrasion resistance and yet retains good clarity, resistance to impact, and toughness.